Vertex Weight Paint Error
Keep in mind that in wireframe view, your strokes are applied to all vertecies behind the brush not just the ones on the closest face. You might try rotataing the model, so only what you want to paint will be hit by the brush, or not using wireframe mode (wireframe mode renders edges as dotted white lines). You can use the "Limit Selection to Visible" button at the bottom toolbar of the viewport to change whether or not your strokes apply to verticies behind the first surface you hit.